App Tracking Transparency Dialog does not appear on iOS

Apple reviewer has just rejected my app since ATT request doesn't appear: "We are unable to locate the App Tracking Transparency permission request when reviewed on iOS 15.0.1."

My code is as shown below:

if #available(iOS 14, *) {
  ATTrackingManager.requestTrackingAuthorization { (status) in
    //print("IDFA STATUS: \(status.rawValue)")
    FBAdSettings.setAdvertiserTrackingEnabled(true)
  }
}

I implemented this code both in AppDelegate didFinishLaunchingWithOptions and viewDidLoad. ATT permission request appear on iOS 14, but not in iOS 15.

Damn it I fixed it:( This is all about the iOS alert system. I was requesting App Tracking Transparency after a notification request was asked. Once the notification request alert closed, the ATT alert needed to have appeared. It was working fine on iOS 14, but on iOS 15 to display an alert right after another one, it is needed to have a delay period between each other.

Edit: Here is my code that display two alert respectively:

 func setNotification(){
    //Ask for notification permission
    let n = NotificationHandler()
    n.askNotificationPermission {
        //n.scheduleAllNotifications()
        
        //IMPORTANT: wait for 1 second to display another alert
        DispatchQueue.main.asyncAfter(deadline: .now() + 1) {
            if #available(iOS 14, *) {
              ATTrackingManager.requestTrackingAuthorization { (status) in
                //print("IDFA STATUS: \(status.rawValue)")
                //FBAdSettings.setAdvertiserTrackingEnabled(true)
              }
            }
        }
    }
}

And for your convenience here is my NotificaitionHandler class:

import UserNotifications

class NotificationHandler{
//Permission function
func askNotificationPermission(completion: @escaping ()->Void){
    
    //Permission to send notifications
    let center = UNUserNotificationCenter.current()
    // Request permission to display alerts and play sounds.
    center.requestAuthorization(options: [.alert, .badge, .sound])
    { (granted, error) in
        // Enable or disable features based on authorization.
        completion()
    }
}